Small Twisted Galaxy Warped by Galactic 'Stealth Merger'

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A small newfound galaxy with an oddball shape may be the product
of a close encounter with a neighboring galaxy in what
scientists are calling a "stealth merger," a new study reveals.

The cosmic discovery came as astronomers investigated an isolated
dwarf galaxy known as NGC 4449, which is about 12.4 million
light-years away and is a "starburst galaxy," meaning it forms
young stars at a furious pace. This galaxy has a distorted
shape as well, a look that is wreathed in hydrogen gas abounding
with rings, shells and a core spinning the opposite way of the
galaxy. Researchers have suspected the galaxy's hyperactivity and
complexity might be due to it tangling
with another galaxy.

Now, two different teams of scientists have detected the most
likely galactic interloper - a previously unseen dwarf galaxy
about 29,300 light-years from NGC 4449. Dubbed NGC 4449B, it is
the largest dwarf galaxy known in the "local group" that includes
the Milky Way and the Andromeda galaxy.

"This is how galaxies grow," said astronomer Aaron Romanowsky of
the University of California, Santa Cruz, a co-author in one of
two separate studies on the interacting galaxies. "You can see
the smaller galaxy coming in and getting shredded, eventually
leaving its stars scattered through the halo of the host galaxy."

The researchers dubbed the interaction a "stealth merger" because
the smaller galaxy NGC 4449B was nearly imperceptible, but has a
profound effect on the shape of its partner.

Romanowsky and his colleagues detail their findings in a study to
appear in Astrophysical Journal Letters. They used telescopes in
California and Hawaii to photograph the two galaxies.

A separate study on the discovery of NGC 4449B appears in the
Feb. 9 issue of the journal Nature.

A twisted dwarf galaxy

NGC 4449B has a twisted shape too, and is essentially a gigantic
"S"-shape so large that if one end were placed at the center of
the Milky Way, the other end would reach all the way to the sun's
position. This oddball shape hints that it was engaged in a
gravitational tug-of-war at some point, researchers said.

"It has clearly suffered a very violent shock and may well be on
the way to being dissolved," said Michael Rich, an astronomer at
the University of California, Los Angeles and lead author of the
Nature study.

"The dwarf is quite interesting — it seems it encountered NGC
4449 on an almost comet-like or 'plunge' orbit," Rich told
SPACE.com.

Galactic surprises

The images of NGC 4449 revealed other surprises as well.

For instance, the galaxy has a strange arc of stars that may
be another
ingested galaxy, and a halo of old stars that appears to
consist of two parts, the outermost part of which makes NGC 4449
roughly the size of the Milky Way. The origin of these old stars
remains uncertain, but the starburst galaxy may have acquired
them when other galaxies like NGC 4449B fell into the galaxy and
were shredded, Rich said.

Scientists have seen other disrupted
galaxies before, such as the Sagittarius dwarf galaxy
near the Milky Way, which has "a trail of stars wrapping at least
twice around our galaxy." However, those other galaxies are much
further along in their destruction.

In contrast, NGC 4449B appears to have only had its first
encounter with its companion, helping shed light on what the
early stages of such entanglements are like.

"I plan to survey more galaxies in an effort to find similar
objects," Rich said.

This story was updated to include more details on the
interactions between the two galaxies based on the second study
to appear in Astrophysical Journal Letters.Visit SPACE.com for
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